In fabrication of VLSI integrated circuits, the use of local interconnects provides for direct local connection between, for example, the source and drain junctions and/or gate electrodes of transistors, without additional contacts and metal layers, and allows for more compact layouts and circuit design. With the widespread use of self-aligned silicidation for source and drain contact metallization, a conductive film of a refractory metal nitride, for example titanium nitride (TIN), is a preferred material for local interconnects. Whereas silicides allow diffusion of boron and phosphorus, resulting in interdiffusion and counterdoping problems, a thin conductive film of TiN, or other refractory metal nitride such as tungsten nitride, is also an effective diffusion barrier.
For example, a known process for forming local interconnects with TiN may include the following process steps:
a. deposition overall of a titanium layer; PA0 b. self aligned silicidation of exposed source, drain, and gate regions in a nitrogen atmosphere to form titanium silicide with overlying layer of titanium nitride (TIN); PA0 c. removal of the TiN layer formed during silicidation; PA0 d. deposition of a layer of conductive TiN; PA0 e. coating and patterning of photoresist to mask selected areas of TiN for interconnect; and PA0 f. etching of the exposed titanium nitride layer to leave TiN interconnects in selected areas.
A conductive titanium nitride film for interconnect may comprise stoichiometric TiN, but the composition may also contain oxygen and/or other elements. Thus, in this application, titanium nitride is denoted by the formula TiN to reflect a composition of approximately 50% titanium with nitrogen, and is not intended to be limited only to stoichiometric TiN. An etch process for defining the local interconnects must have selectivity to interconnect material, e.g. TiN, relative to underlying silicide to prevent degradation of the latter during etching. Commonly used silicides include TiSi.sub.2, CoSi.sub.2, and PtSi.sub.2. Etch selectivity is also required relative to other substrate materials including oxides, photoresist encountered in semiconductor processing.
It is well known that a simple halogen containing discharge, and in particular a fluorine discharge, is effective for reactive ion etching of refractory metal nitrides, particularly TiN. However, a halogen discharge does not provide etch selectivity relative to TiSi.sub.2 and other refractory metal silicides, such as WSi.sub.2 and MoSi.sub.2 or
Various processes are known which have sought to provide improved etch selectivity for titanium nitride relative to silicides and other integrated circuit substrate materials using plasma or reactive ion etching with fluorine containing gases.
For example, a known method of selectively etching TiN relative to titanium silicide is disclosed in U.S. Pat. No. 4,675,073 to Douglas, which describes a dry etch process using a conventional plasma etching system providing a glow discharge plasma containing reactive fluorine species, provided by a feed gas comprising CF.sub.4 in helium. The process takes advantage of the passivation effect of adsorption of CF.sub.2 .cndot. radicals, formed by dissociation of CF.sub.4, which are preferentially adsorbed on the silicide so as to hinder etching of the silicide by fluorine radicals. However, it was found that selectivity between TiN and TiSi.sub.2 is low, and both oxide and photoresist were rapidly etched. It was also found that the high energy ions generated in a conventional plasma etch system, with ion energies typically .about.100-250 eV, may cause surface damage to exposed silicide and other substrate materials during overetching near the end point. Thus, to avoid substrate damage and improve control of the endpoint, a dry etch step using a fluorine plasma is typically used to etch about 90% of the total thickness of TiN, and the remaining thickness of TiN is removed to the endpoint, with a subsequent wet etch. The latter comprises, for example an aqueous solution of dilute H2O.sub.2 and NH.sub.4 OH. Nevertheless, this wet etch solution also attacks photoresist, necessitating stripping of photoresist prior to the wet etch. Consequently, the wet etch step is not selective and after removing the photoresist the overall thickness of the exposed TiN, i.e. that for forming the interconnect, is also reduced during the wet etch. Furthermore, while NH.sub.4 OH/H.sub.2 O.sub.2 is suitable for stripping or cleaning TiN, the etching rate is not sufficiently controllable for repeatably removing a small thickness of TiN.
Other fluorine containing feed gases, for example, CHF3, C2F6 and SF6, have been used for non-selective dry etching of titanium nitride, as described in U.S. Pat. No. 4,877,482 to Knapp et al. However, the '073 patent to Douglas teaches that a gas which is a copious fluorine source, e.g. SF.sub.6, is unsuitable for selective etching, because the etch selectivity of TiN relative to TiSi2 is reduced in the presence of excess fluorine. Consequently, Douglas found that it was advantageous (see Douglas '073 patent discussed above) for improved selectivity to use a low flow rate of a non-copious fluorine source gas, i.e. CF.sub.4 in helium with a reducing electrode to scavenge fluorine and maintain fluorine deficiency in the plasma.
In two other recent U.S. Pat. Nos. 4,863,559 and 4,793,896 to Douglas, a method is described which provides improved etch selectivity using a plasma photo-generated from a gas mixture of CCl.sub.4 and helium, in which excess chlorine is reduced by use of a consumable power electrode or by introducing a chlorine scavenger gas such as chloroform into the reactor.
In U.S. Pat. No. 4,878,994 to Jucha, there is described another chlorine based process for selective etching of Ti containing film, such as TiN, with respect to silicide which utilizes a plasma generated from a helium and CCl.sub.4, in a plasma etching system which provides for a two stage plasma generation process to improve control of the etch anisotropy, selectivity and etch rate, compared to the method of the Douglas '073 patent, so that a subsequent wet etch step is unnecessary.
However, in use of a chlorine bearing gas for etching TiN, the reaction product is titanium chloride, which is less volatile than the corresponding fluoride. Thus a fluorine plasma is preferred to reduce residues and increase etch rate. There may also be problems associated with residual interfacial deposits left on surfaces parallel to the ion flux and caused by chlorine plasma reactions with SiO.sub.2. Further, plasma etching with CCl.sub.4 causes increased polymerization of resist material which necessitates a special resist strip. Use of a graphite electrode to scavenge chlorine may result in carbon contamination and further polymerization. Thus, it is preferred for microelectronics applications to avoid an etchant gas containing reactive chlorine species, which may result in contamination problems, as well as for reasons of health and environmental concerns associated with chlorinated gases.
Thus although the use of refractory metal nitrides as interconnect materials provides a significant advantage in simplifying circuit layout in VLSI integrated circuits, it is believed that the lack of selectivity of known etch processes may be a significant factor limiting more widespread use of refractory metal nitride local interconnects.